There are generally three types of stepping motors: the variable reluctance-type, the hybrid-type, and the permanent-magnet type. With an appropriate driver (i.e., controller), all three types offer the capability of a wide range of angular stepping or indexing movements and characteristics. A general reference on control of stepping motors can be found on-line at http://www.cs.uiowa.edu/˜jones/step/ by Douglas W. Jones. The present invention concerns permanent magnet-type stepping motors.
Both hybrid-type and permanent magnet-type stepping motors use permanent magnet(s) in the moving (e.g., rotor) and stationary (e.g., stator) structures. They can be indistinguishable from the motor driver's point of view. Traditionally, the rotor of a hybrid-type stepping motor is built with a donut-shaped magnet at the center of two rotor poles, resulting predominantly in axial flux flow from the magnet to the two rotor poles. Various stepping motor designs involving permanent magnets are derived from magnetic circuit manipulation of poles and magnets. The permanent magnet-type stepping motors with which this invention is principally concerned are representatively shown and described by Schaeffer in U.S. Pats. No. 4,190,779 and 4,315,171, the aggregate disclosures of which are hereby incorporated by reference.
The permanent magnet-type stepping motors disclosed by Schaeffer have a large number of stator teeth and a large number of radially-magnetized magnets on the rotor to provide for small stepping angles. These motors have the advantage of high unpowered and powered detent torques, relatively-short axial motor lengths (i.e., pancake style), small rotor inertias, and large through-hole solutions on the rotor. These motors have found great success in the last thirty years in space applications, such as in powering solar array drives and antenna pointing mechanisms. Such applications have required light masses, high powered and unpowered detent torques, small rotor inertias, large shafts and/or large numbers of harness feed-throughs on the rotor.
Advanced applications require state-of-the-art stepping motor designs with more torque, reduced size and mass, higher torque density, smaller rotor inertia, reduced cost, constant peak powered and unpowered detent torques, and constant peak running torques. Inherent in a stepping motor design is the concern of maintaining synchronization, that is, the ability of the motor and output load to maintain rotation together in response to every motor input command signal. Given that there is typically no feedback to assure that this synchronous operation of the stepping motor and load occurs, the stepping motor's output is dependent not only on the torque production capability, but also on the motor's stability of action in response to step commands while driving the load. Stepping motors operate open-loop, and so system concerns such as resonances may be mitigated through enhanced motor stability. Hence, the necessity exists to improve both torque density and step stability in order to improve upon state-of-the-art stepping motor technology.